There are two methods to procure Sith Armor, both of which are in the Upper City North Apartments:

Head into the North Apartments. In one of the rooms there will be two Sith Troopers berating and yelling at an alien, saying he's a spy and threatening to take him in. You can kill the troopers, and after talking to the alien (who admits he is actually a member of the Hidden Beks), you can loot the armor off of the soldiers.

Talk to Sarna in the Upper City Cantina. Empathize with her about being stuck on Taris, and she will invite you to a party in the Upper City North Apartments. When you arrive, she happily greets you can goes back to the party, where soon everyone passes out on the floor, completely smashed. You can then loot the Armor out of the backpack in the corner.